An internal combustion engine with a fuel injector may require a separate injector sleeve insert to separate coolant from the fuel injector. Many designs for injector sleeve insertion exist with varying degrees of robustness against coolant, fuel, and combustion gas, leaks, particularly at the end closest to the combustion event, i.e. the combustion chamber. The high local temperatures make elastomeric sealing a challenge. Also, high mechanical and thermal load cycling may create high stress at the sleeve/head seal interface. Various conventional sleeve and cylinder head designs possess various complexities in the cylinder head to satisfy long term cylinder head durability requirements, and these complexities sometimes involve expensive details requiring tight tolerance and process controls.